1. Field of the Invention
The present invention relates to a streak tube capable of measuring a high-speed time-dependent change in the brightness of light within several hundred femtoseconds. More particularly, the invention relates to a streak tube having an arrangement for suppressing travel time spread of photoelectrons caused by difference in energy in each photoelectron emitted from a photocathode.
2. Description of Prior Art
Streak tubes are devices for converting a time-dependent intensity distribution of light to be measured into a spatial intensity distribution on an output plane. Since the streak tubes have a picosecond time resolution, they are used for an analysis of the phenomenon of light at ultrahigh speed.
A conventional streak tube has a structure as shown in FIGS. 1A, 1B, 2A and 2B of the accompanying drawings.
FIG. 1A is a cross-sectional view showing the streak tube, taken along a plane parallel to deflection electrodes, and FIG. 1B is a diagram showing the relationship between a photocathode and an optical image formed thereon in the streak tube shown in FIG. 1A. FIG. 2A is a cross-sectional view showing the streak tube, taken along a plane including the axis of the streak tube and perpendicular to the deflection electrodes, and FIG. 2B is a diagram view showing the relationship between a photocathode and an optical image formed thereon in FIG. 2A. As shown in FIGS. 1A, 1B, 2A, and 2B, the streak tube, generally denoted by reference numeral 1, includes a hermetic vacuum casing 2 which has an input window 3 on one end of the casing 2, for focusing thereon an optical image to be analyzed, and an output window 4 on the other end of the casing 2, for emitting the processed optical image out of the casing 2.
Between the input and output windows 3, 4, there are successively disposed, along the axis of the streak tube 1, a photocathode 5, an acceleration mesh electrode 6, a focusing electrode 7, an aperture electrode 8, deflection electrodes 9, and a phosphor screen 10. Progressively higher voltages are applied to the focusing electrode 7, the mesh electrode 6, and the aperture electrode 8 in the stated order with respect to the photocathode 5. The same potential as that of the aperture electrode 8 is applied to the phosphor screen 10.
An optical image 11 is projected from a device (not shown) onto the photocathode 5 through the input window 3 on a line passing through the center of the photocathode 5. The photocathode 5 then emits an image of electrons corresponding to the optical image. The emitted electrons are accelerated by the mesh electrode 6, focused by the focusing electrode 7, pass through the aperture electrode 8, and enter the gap between the deflection electrodes 9.
While the linear electron image is passing through the gap between the deflection electrodes 9, a ramp deflection voltage is applied between the deflection electrodes 9 to produce a deflection electric field that deflects the electron image. The deflected electron image is applied to the phosphor screen 10.
At this time, the electric field generated by the deflection voltage is directed perpendicularly to both the tube axis and the linear electron image, i.e., perpendicularly to the sheet of drawing in the case of FIG. 1A, and parallel to the sheet of drawing in the case of FIG. 2A. The intensity of the electric field is proportional to the deflection voltage. The linear electron beam is scanned in a direction perpendicular to the longitudinal direction of the linear electron beam and an optical image, referred to as a streaked image, is formed on the phosphor screen 10. The streaked image is an array of images that are in the time domain of the linear optical image on the photocathode 5 and are arranged in the direction perpendicular to the linear optical image. Therefore, any change in the brightness of the streaked image along its image array, i.e., in the direction in which it is swept, is representative of a time-dependent change in the intensity of the optical image 11.
In the streak tube, the photoelectrons emitted from the photocathode have various energies. Therefore, the photoelectrons that have simultaneously been emitted from the photocathode 5 reach the deflection electrodes 9 at different times, resulting in a travel time spread of the photoelectrons. The travel time spread is partly responsible for a limited time resolution of the streak tube 1.
Generally, the energy distribution of the photoelectrons emitted from the photocathode is determined by the type of the photocathode and the wavelength of the light to be measured, and the acceleration of the photoelectrons is determined by a distribution of potentials along the tube axis from the photocathode to the deflection electrodes. Consequently, the travel time spread is determined by the type of the photocathode, the wavelength of the light to be measured, and the potential distribution along the axis of the streak tube.
Heretofore, in order to reduce the travel time spread, the acceleration mesh electrode is disposed in proximity with the photocathode to accelerate the photoelectrons quickly for thereby minimizing a region in which the photoelectrons travel at low speed in the vicinity of the photocathode. The travel time spread between the photocathode and the mesh electrode is determined by only the electric field therebetween once the type of the photocathode used and the wavelength of the light to be measured are given.
FIG. 3 shows the relationship between the electric field and the travel time spread when the wavelength of the light to be measured is 500 nm with the use of a photocathode S-20 according to the standards of Electronic Mechanical Industrial Association of the United States. Study of FIG. 3 indicates that theoretically, the travel time spread can be reduced to any desired level if the electric field is increased. Actually, however, when the surface of the photocathode is in a potential of 6 kV/mm or higher, the photocathode emits a dark current due to the field emission effect even if no incident light is applied to the photocathode, thereby increasing noise-induced background emission on the output phosphor screen and thus degrading a signal-to-noise ratio.
If the photocathode has a minute surface projection thereon, then there is developed a very strong electric field on the surface of the photocathode. The photocathode produces a very large dark current due to the tunnel effect, and the dark current induces a white spot on the output phosphor screen.
The background emission on the output phosphor screen may be reduced by applying a voltage between the photocathode and the mesh electrode for a very short period of time, thereby reducing the time in which any dark current is generated. To this end, a short pulse voltage as shown in FIG. 4, for example, may be applied to the photocathode through flange electrodes which support the input window on which the photocathode is mounted.
However, inasmuch as a portion of the applied pulse voltage wave is reflected because of the lack of impedance matching, or owing to a high electrostatic capacitance between the flange electrodes, a reduced voltage whose waveform is less sharp as shown in FIG. 5 is applied to the photocathode. As a consequence, it is not possible to apply a pulse voltage of required magnitude, and an acceleration voltage is applied between the photocathode and the acceleration electrode for a period of time longer than the duration of the pulse voltage that has been generated by a pulse voltage power supply. The initially intended application of a voltage for a short period of time for the reduction of the time in which a dark current is generated, cannot therefore be achieved.
Moreover, the distance between the photocathode and the acceleration electrode in the conventional streak tube cannot be reduced to 0.5 mm or less. The distance therebetween is set in such a manner that upon interposing a spacer of a predetermined thickness between the photocathode and the acceleration electrode, the latter is welded to supporting portions extending from the outer wall of the streak tube. With such a setting, a high assembling accuracy cannot be attained due to the deformation of the electrode resulting from the presence of the spacer and the welding.
If the travel time spread of the photoelectrons between the photocathode and the acceleration electrode is to be suppressed within 50 fs or shorter, then a pulse voltage of 36 kV/mm or 18 kV/0.5 mm is required to obtain a relevant electric field. However, it is extremely difficult to produce such a high voltage with a very short pulse duration of 1 ns, for example.